If you want to go into the highlands and the interior of Iceland, then the best time to go is from the end of June to the end of September.
The first tracks open usually in the beginning of June and the last in the end of June.
The weather is also better in the summer time than the winter time, with light nearly all around the clock in June and July.
When visiting Iceland in the winter time, will be completely different. The colors are not the same, but you will see frosen waterfalls which is magic and if you are lucky the northern lights.
The interests are definitely not the same when travelling in the summer and in the winter
Short weekend breaks are perfects in the winter time, enjoying the hot springs and spas around Reykjavik.
Doing a ski trip to Iceland is also ideal in the winter time.
Icelandic Roads
Iceland has different kind of roads,asphalt roads, gravel roads and tracks but no highways. The ringroad (Road n° 1) is around 1300 km, interior roads are tracks where you will go only with 4x4 wheel drive vehicules. They are often narrow.
The general speed limit is 50 km/h in urban areas, 80 km/h on gravel roads in rural areas, and 90 km/h on asphalt roads.